Impurity in Tree Classifiers
“The first problem in tree construction is how to use L to determine the binary splits of X into smaller and smaller pieces. The fundamental idea is to select each split of a subset so that the data in each of the descendant subsets are ‘purer’ than the data in the parent subset…the node impurity is largest when all classes are equally mixed together in it, and smallest when the node contains only one class.” (pp. 23-24)